The present invention relates to a carbon material and, more particularly, to a method for preparing a granular graphite material which is of light weight, is heat resistant and excels in elasticity.
In general, carbon materials have such properties as not found in other materials, as typically expressed in terms of light weight, heat resistance and high elasticity. They are rigid in block form, even though they are porous, whereas they are deformable in fine or thin form, e.g., in fiber, sheet or film form. However, such carbon materials do not have the property of being elastic in themselves. As carbon material of elasticity, there have heretofore been known sheets prepared from expanded graphite. However, their elastic recovery is low, as measured when compression load is applied and thereafter removed. (See Saito "Industrial Materials", Vol. 29, page 34, 1985).
The present inventors have already provided a method for preparing granular elastic graphite materials having an increased elastic recovery, as determined when compression load is applied and thereafter removed (Japanese Patent Laid-Open Publication Nos. 13908/1988 and 9808/1989). According to the methods disclosed therein, a carbonaceous mesophase or raw coke treated with nitric acid or a mixed acid of nitric acid with sulfuric acid (as described in Japanese Patent Laid-Open Publication No. 285493/1986) or a carbonaceous mesophase or raw coke treated with nitric acid or a mixed acid of nitric acid with sulfuric acid (as disclosed in Japanese Patent Laid-Open Publication No. 9808/1989) is dissolved in an aqueous alkaline solution and precipitated in an aqueous acid solution to obtain an aqua-mesophase, which is in turn treated by heating at about 300.degree. C. for expansion and foaming and then graphitized at a temperature of 2400.degree. C. or higher. It is true that the graphite materials obtained by such methods have excellent elastic recoveries, but it is required that, in the former method, after treated with nitric acid or a mixed acid of nitric acid with sulfuric acid or with nitric acid or a mixed acid of nitric acid with sulfuric acid in the latter method, the thus acid treated material be further treated for dissolution and precipitation. The treatment with nitric acid or a mixed acid of nitric acid with sulfuric acid, common to both methods, is carried out to simultaneously introduce into the starting material a nitro group and a carboxylic acid group and a hydroxyl group, both formed by oxidation that is a side reaction. From this, it is presumed that the expansion and foaming phenomena occurring during the heat treatment at about 300.degree. C. are brought about by the melting of the raw material by the nitro group and gases generated due to the decomposition of the carboxylic acid and hydroxyl groups. In other words, it appears that the presence of the nitro group, carboxylic acid and the like are an inevitable requirement for such methods.
In any case, such conventional methods as mentioned above involve relatively sophisticated steps and operations and are thus time consuming.